Medical devices and wearable consumer products have fundamental anatomically-driven size constraints that necessitate small form factors. Since most patients and consumers desire long battery life, and battery volume is limited by anatomy, one of the only ways to increase lifetime is to reduce the power of the underlying circuits. Even when wireless communications are limited in distance or duration to save power, the energy budget of the wireless communication components of the device can still dominate the overall energy budget of a wearable device.
Most efforts have therefore focused on providing higher performance wireless circuits. Low power, high performance wearable circuits tend to use expensive components. Ultra-low power radio circuits for example are available from IMEC as custom circuit designs, but depend up leveraging very small node low-power CMOS transceivers, e.g. a 7 Gbps 60 GHz transceiver IC implemented in 40 nm low-power CMOS. The cost of such wireless transceivers can substantially raise the price of a wearable component, and there is still a desire for reduced cost, area and power consumption wireless communications to improve wearable medical and consumer body monitoring devices.
One approach that turned away from merely improving the circuit efficient and power performance of conventional transceivers is an approach that uses the human body as a communication channel for electric fields via galvanic coupling. An e-textile approach was developed by one of the inventors and a colleague. See, P. P. Mercier and A. P. Chandrakasan, “A Supply-Rail-Coupled eTextiles Transceiver for Body-Area Networks,” IEEE J. Solid-State Circuits, vol. 46, no. 6, pp. 1284-1295, June 2011. Others have also used the human body as a communication channel for electric fields. Song, S. Lee, N. Cho, and H. Yoo, “Low Power Wearable Audio Player Using Human Body Communications,” in 2006 10th IEEE International Symposium on Wearable Computers, 2006, pp. 125-126. The eTextiles offers the lowest power consumption due to inherently low path loss, but leveraged dedicated clothing, which may not be practical or desirable in many applications.
Galvanic coupling typically employs two electrode pairs, which can be attached on the skin as the transmitter (TX) and receiver (RX) nodes. At the TX node, an electrical signal is applied differentially, inducing small currents that propagate across the entire body, some of which can be sensed by the RX. Thus, galvanic coupling acts much like a distributed wired connection across the body, and can thereby achieve a high level of security/privacy and good interference resiliency.
Another approach relies upon electric field human body communication and can be referred to as eHBC. J. H. Hwang, T. W. Kang, S. O. Park, and Y. T. Kim, “Empirical Channel Model for Human Body Communication,” IEEE Antennas Wirel. Propag. Lett., vol. 14, pp. 694-697, 2015. Such systems can have lower path loss compared to conventional far-field radios (e.g., Bluetooth, WiFi, Zigbee, LTE, etc.), and further benefit from lower-complexity multi-user access and security requirements due to limited broadcasting of energy. However, the improvement in path loss is not always large, especially when small, battery-powered devices are used, and thus the advantages of eHBC over conventional radios is still unclear. Additionally, galvanic eHBC systems have limited dynamic path loss degradation due to movement, and can be used to communicate with implants. However, due to the low conductivity of tissues found in the human body, galvanic eHBC has relatively large path loss compared to other approaches. B. Kibret, M. Seyedi, D. T. H. Lai, and M. Faulkner, “Investigation of galvanic-coupled intrabody communication using the human body circuit model,” IEEE J. Biomed. Heal. informatics, vol. 18, no. 4, pp. 1196-206, July 2014.
Other systems capacitively couple to the body. Capacitive eHBC systems also require two electrode pairs to generate differential signals around the human body, but their physical configurations are slightly different. With a capacitive couple, only one electrode should be directly placed on (or near) the skin to produce electric fields within the human body, while the other electrode should be placed facing outwards in order to capacitively couple to the environment. See, e.g., T. G. Zimmerman, “Personal area networks (PAN): Near-field intra-body communication,” M. S. Thesis, Massachusetts Inst. Technol., Cambridge, Mass., 1995. According to other researchers, this coupling mechanism can be modeled as distributed RC circuits if the operation frequency is low enough for electrostatic analysis. N. Cho, J. Yoo, S. J. Song, J. Lee, S. Jeon, and H. J. Yoo, “The human body characteristics as a signal transmission medium for intrabody communication,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 5, pp. 1080-1085, 2007. However, as the frequency is scaled above tens of MHz, the power radiated by electrodes increases, and others have proposed a wave propagation model operating on the surface of human body. J. Bae, H. Cho, K. Song, H. Lee, and H.-J. Yoo, “The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication,” IEEE Trans. Microw. Theory Tech., vol. 60, no. 3, pp. 582-593, March 2012. Such models have shown that capacitive coupling achieves a lower path loss than galvanic coupling.
However, the present inventors have recognized that capacitive eHBC systems suffer from a number of drawbacks. For example, they require large ground planes to increase environmental coupling and reduce path loss. This path loss also has high variability based on environmental conditions (and the availability of objects to couple to). Furthermore, since the IEEE established the 802.15.6 WBAN standard in 2012, eHBC has used 21 MHz as its operation frequency, yet capacitive coupling generally achieves the lowest path loss at higher frequencies. The present inventors have also identified that although capacitive eHBC can offer superior path loss compared to conventional far-field radiation, the variance caused by environmental effects and the poor conductivity of the human body further limit its utility.